i

NOVEL RADIATION GRAFTED MEMBRANES BASED ON FLUORINATED POLYMERS

FOR PROTON EXCHANGE MEMBRANE FUEL CELL

by

SAHL SADEGHI

Submitted to the Graduate School of Engineering and Natural Sciences

in partial fulfillment of the requirements for the degree of Master of Science

Sabanci University

August 2016

ii

NOVEL RADIATION GRAFTED MEMBRANES BASED ON FLUORINATED POLYMERS

FOR PROTON EXCHANGE MEMBRANE FUEL CELL

APPROVED BY:

Assoc. Prof. Dr. Selmiye Alkan Gürsel

(Thesis Supervisor)

Assoc. Prof. Dr. Gozde Ince

Asst. Prof. Dr. Oktay Demircan

DATE OF APPROVAL: 9 Aug 2016

iii

© Sahl Sadeghi 2016

All Rights Reserved

iv

NOVEL RADIATION GRAFTED MEMBRANES BASED ON FLUORINATED POLYMERS

FOR PROTON EXCHANGE MEMBRANE FUEL CELL

SAHL SADEGHI

MAT, M.Sc. Thesis, 2016

Thesis Supervisor: Assoc. Prof. Selmiye Alkan Gürsel

ABSTRACT

In the first part of this thesis a facile method for preparing poly(vinylidene fluoride) (PVDF)-g-

poly(styrene sulfonate acid) (PSSA) membranes by radiation induced graft polymerization is reported.

Sodium styrene sulfonate (SSS) monomer has been used for the grafting of SSS from PVDF powder in

aqueous dimethyl sulfoxide (DMSO) solution, and it precipitated after synthesis. Later on, the resultant

PVDF-g-PSSS graft copolymer membranes were prepared by means of vapor induced phase separation

(VIPS) technique at 60% relative humidity (RH), and dried under vacuum at high temperature to achieve

PVDF-g-PSSA proton conducting nano-porous membranes. It was found that these membranes exhibit

encouraging results in terms of higher conductivity and better mechanical properties compared to Nafion®

NR-211.

In the second part of thesis, the effect of divinylbenzene (DVB) as a cross-linker on the graft

polymerization of 4-vinylpyridine (4-VP) from poly(ethylene-co-tetrafluoroethylene) (ETFE) films was

studied. The resulted films were doped with phosphoric acid (PA), and examined for mechanical

properties and fuel cell performance. The cross-linked membrane obtained from grafting a mixture of 4-

VP with 1% DVB improved the polymerization kinetics, and resulted in 50% graft level (GL). The

resulted membrane additionally exhibited proton conductivity as high as 75 mS/cm at 50% relative

humidity and 120 °C, besides doubling the power output of fuel cell comparing to a non-cross-linked

membrane.

v

PROTON DEĞĠġĠM MEMBRANLI YAKIT HÜCRESĠ ĠÇĠN FLORÜRLÜ POLĠMER ESASLI

RADYASYONLA AġILANMIġ ÖZGÜN MEMBRANLAR

SAHL SADEGHI

MAT, M.Sc. Tezi, 2016

Tez danıĢmanı: Doç. Dr. Selmiye Alkan Gürsel

ÖZET

Tezin ilk bölümünde polistiren sülfonik asit esaslı polivinilidin florür (PVDF-g-PSSA)

membranlarının radyasyonla aĢılama polimerizasyonu ile hazırlanması açıklanmıĢtır. Sodyum

stiren sülfonat monomeri toz haldeki PVDF ile dimetil sülfoksit ortamında aĢılanmasında

kullanılmıĢ ve sentez sonrası çökelmesi sağlanmıĢtır. Daha sonra, PVDF-g-PSSS kopolimer

membranları buhar esaslı faz dönüĢüm tekniği ile %60 bağıl nemde sentezlenmiĢ, sıcak vakum

ortamında kurutularak proton değiĢimli nano-gözenekli yapı elde edilmiĢtir. Üretilen bu

membranların ticari Nafion® NR-211 membranına kıyasla yüksek iletkenlik ve iyileĢtirilmiĢ

mekanik özellikleri açısından ümit verici özellikler sergilediği gözlemlenmiĢtir.

Tezin ikinci bölümünde, divinilbenzen (DVB) çapraz bağlayıcısının polietilen tetrafloroetilen

(ETFE) filmleri üzerinde 4-vinilpiridin (4-VP) ile polimerleĢerek aĢılanması üzerinde

durulmuĢtur. AĢılanmıĢ filmler fosforik asit ile katkılandırıldıktan sonra mekanik özellikleri ve

yakıt hücresi performansı bakımından karakterize edilmiĢtir. 4-VP ve %1 DVB içeren aĢılama

çözeltisiyle üretilmiĢ çapraz bağlı membranlar, ileri polimerizasyon kinetikleriyle %50 aĢılama

derecesi sağlamıĢtır. Üretilen çapraz bağlı membranların 120°C sıcaklığında %50 bağıl nemde

75 mS/cm gibi yüksek iletkenlik göstermesinin yanı sıra çapraz bağlı olmayan membranlara göre

iki kat daha fazla güç yoğunluğu sağlamıĢtır.

vi

“Education is not preparation for life; education is life itself.”

- John Dewey

vii

ACKNOWLEDGEMENTS

I would like to extend my sincerest appreciation and thanks to my supervisor professor Dr.

Selmiye Alkan Gursel. I would like to thank you for encouraging my research and for allowing

me to grow as a research scientist. I would also like to thank Prof. Dr. Gozde Ince and Prof. Dr.

Oktay Demircan for serving as my committee members, and for your brilliant comments and

suggestions. I would also like to take the chance to appreciate my instructors in faculty of

engineering and natural science of Sabanci university who patiently helped me to extend my

knowledge in my field of research. My special thanks go to my colleagues, friends and

researchers at SUNUM facility, especially my mentors Dr. Enver Guler, Dr. Veera Sadhu, Dr.

Lale I. Sanli who supported me as friends.

viii

TABLE OF CONTENT

ABSTRACT ................................................................................................................................... iv

ÖZET .............................................................................................................................................. v

ACKNOWLEDGEMENTS .......................................................................................................... vii

TABLE OF CONTENT ............................................................................................................... viii

LIST OF FIGURES ....................................................................................................................... xi

LIST OF TABLES ....................................................................................................................... xiv

LIST OF EQUATION .................................................................................................................. xv

ABBREVIATIONS AND SYMBOLS ........................................................................................ xvi

1. INTRODUCTION ...................................................................................................................... 1

1.1. History of fuel cell ............................................................................................................................. 1

1.2. Types of fuel cells .............................................................................................................................. 2

1.2.1. Low temperature fuel cells .......................................................................................................... 3

1.2.2. Intermediate temperature fuel cells ............................................................................................. 5

1.2.3. High temperature fuel cells ......................................................................................................... 6

1.3. Governing principles for proton exchange fuel cell ........................................................................... 7

1.4. Proton exchange membranes ........................................................................................................... 14

1.4.1. Fluorinated proton exchange membranes ..................................................................................... 16

1.4.2. Sulfonic polymers and processes .................................................................................................. 19

1.4.3. Radiation induced graft polymerization ........................................................................................ 20

2. NANO-STRUCTURED POLY(VINYIDENE FLUORIDE) GRAFT POLYSTYRENE

SULFUNIC ACID FOR PROTON EXCHANGE MEMBRANE ............................................... 23

2.1. Introduction ...................................................................................................................................... 24

2.2. Experimental .................................................................................................................................... 26

ix

2.2.1. Material ..................................................................................................................................... 26

2.2.2. Radiation induced graft copolymerization ................................................................................ 26

2.2.3. Membrane preparation .............................................................................................................. 27

2.2.4. Characterization of membranes ................................................................................................ 27

2.3. Results and discussion ..................................................................................................................... 28

2.3.1. Graft level ................................................................................................................................. 30

2.3.2. Water up-take ............................................................................................................................ 31

2.3.3. Proton conductivity ................................................................................................................... 32

2.3.4. Thermogravimetric analysis ...................................................................................................... 34

2.3.5. Mechanical properties ............................................................................................................... 34

2.3.6. Fuel Cell performance ............................................................................................................... 35

2.4. Conclusions ...................................................................................................................................... 36

3. CROSS-LINKED PROTON EXCHANGE MEMBRANES BY RADIATION INDUCED

GRAFTING OF 4-VINYLPYRIDINE AND DIVINYLBENZENE FROM POLY(ETHYLENE-

CO-TETRAFLUOROETHYLENE) FILMS ................................................................................ 37

3.1. Introduction ...................................................................................................................................... 38

3.2. Experimental .................................................................................................................................... 40

3.2.1. Materials ................................................................................................................................... 40

3.2.2. Membrane preparation .............................................................................................................. 41

3.2.3. Characterization of membranes ................................................................................................ 41

3.3. Results and discussion ..................................................................................................................... 42

3.3.1. The effect of reaction time on graft level .................................................................................. 42

3.3.2. The Effect of DVB concentration on graft level ....................................................................... 43

3.3.3. The effect of DVB on phosphoric acid up-take ........................................................................ 44

3.3.4. The effect of DVB on phosphoric acid loss .............................................................................. 45

3.3.5. The effect of DVB on mechanical properties of membranes .................................................... 47

x

3.3.6. Ionic conductivity of grafted membranes ................................................................................. 47

3.3.7. Fuel cell performance of ETFE-g-PVP membranes ................................................................. 48

3.4. Conclusion ....................................................................................................................................... 49

REFERENCES ............................................................................................................................. 51

xi

LIST OF FIGURES

Figure 1. 1. Schematic of Sir William Grove fuel cell 1839 [6]. .................................................................. 1

Figure 1. 2. Classification of current commercial fuel cell technologies [8]. ............................................... 2

Figure 1. 3. Comparison of proton exchange fuel cell (PEFC) and anion exchange membrane fuel cell

(AEMFC) [18]. .......................................................................................................................... 4

Figure 1. 4. Materials and related issues for SOFC [46]. .............................................................................. 7

Figure 1. 5. The effect of temperature vs. cell voltage E°(v) for hydrogen and methane as fuel [73] . ...... 11

Figure 1. 6. Typical performance of a hydrogen fuel cell and the effect of common overpotentials [73].. 13

Figure 1. 7. Semiempirical potential energy for proton transfer across hydrogen bindings of symmetrical

conformations of the type R-O-H...O-R for different oxygen distances Q and full stabilization

of the surrounding [76]. ........................................................................................................... 14

Figure 1. 8. Scheme of the hypothetical mechanism, in which a Grotthuss-type mechanism is occured by a

short-distance transportation of hydronium ions [82]. ............................................................ 15

Figure 1. 9. Proton conductivity system of phosphoric acid-doped PBI (a) phosphoric acid –water

hydrogen ion transfer; (b) benzimidazole – phosphoric acid proton transfer; (c) hopping

between phosphoric acid [27]. ................................................................................................. 16

Figure 1. 10. A) Nafion® B) Aquivion® C) 3M structures [92]. .......................................................... 17

Figure 1. 11. Different synthesis pathways for perfluoro(alkyl vinyl ether) with sulfonyl acid fluoride,

above) DuPont method for Nafion®, below) Solvay method for Aquivion [93]. ................... 18

Figure 1. 12. 3M method for synthesizing perfluoro(alkylvinyl ether) with sulfonyl acid fluoride

monomer [95]. ......................................................................................................................... 18

Figure 1. 13. Schematic of sulfonation of poly(ether ether ketone) (PEEK) by concentrated sulfuric acid

[106]. ....................................................................................................................................... 19

Figure 1. 14. Synthetic methods for preparing comb-like polymers: (a) “Grafting-onto” is the grafting of

functional side chains to a polymeric backbone with active groups. (b) “Grafting-through”

comprises of the polymerization process of functionalized monomers. (c) “Grafting-from” is

the grafting of co-polymerization of vinyl monomers on a polymeric backbone with active

sites [114]. ............................................................................................................................... 20

Figure 1. 15. Plausible mechanism of preparation of phosphoric acid doped poly(4-VP) grafted ETFE

xii

membrane [133]. ..................................................................................................................... 21

Figure 2. 1. 1H-NMR result of PVDF-g-PSSS, “a” peaks belong to PVDF, and “b” and “c” peaks belong

to SSS. ..................................................................................................................................... 30

Figure 2. 2. Graft level of PVDF-g-PSSS with respect to reaction time..................................................... 31

Figure 2. 3. The relation between graft level and water up-take of membranes prepared by VIPS method.

................................................................................................................................................. 32

Figure 2. 4. Proton conductivity of the membranes prepared by tape casting and mold casting. ............... 33

Figure 2. 5. Scanning electron microscopy imaging of sub-micron structure of PVDF-g-PSSS membrane

with 35% graft level. ............................................................................................................... 33

Figure 2. 6. Thermogeravimetric analysis of PVDF-g-PSSA compared to pristine PVDF, PVDF-g-PSSS

and Nafion® NR-211. ............................................................................................................. 34

Figure 2. 7. The universal tensile stress results of fully humidified PVDF-g-PSSA membrane with

different graft levels. ............................................................................................................... 35

Figure 2. 8. Current-voltage and current-power of fuel cell performance of 35% grafted PVDF-g-PSSA at

60°C and 80%RH vs. Nafion® NR-211 at 80°C and 60%RH. ............................................... 36

Figure 3. 1. Mechanism of preparation of cross-linked phosphoric acid doped poly(4VP) grafted ETFE

membrane. ............................................................................................................................... 40

Figure 3. 2. The graft level of ETFE-g-PVP films with 0%DVB and 1%DVB at 60°C, 50 kGy and

varying reaction time. .............................................................................................................. 43

Figure 3. 3. The effect of DVB concentration on graft level of 50 kGy ETFE films at 60°C and 4 hours

grafting time. ........................................................................................................................... 43

Figure 3. 4. The effect of DVB concentration on the phosphoric acid up-take during doping process. ..... 44

Figure 3. 5. Scanning electron microscope image of grafted ETFE films: left) 0%DVB grafted film right)

1%DVB grafted film. .............................................................................................................. 45

Figure 3. 6. The effect of DVB on acid up-take and acid loss of grafted membranes with the same initial

conditions. ............................................................................................................................... 46

Figure 3. 7. The effect of cross-linking on contact angle of acid doped membranes: left) 0% DVB

xiii

mebrane, right) 1% DVB membrane. ...................................................................................... 46

Figure 3. 8. The tensile test results for acid doped membranes with 0%, 1% and 2% DVB content. ........ 47

Figure 3. 9. The proton conductivity of ETFE-g-PVP membranes at different relative humidity and

temperature. ............................................................................................................................. 48

Figure 3. 10. Fuel cell performance of ETFE-g-PVP at 50%RH, 1atm and different temperatures. .......... 49

xiv

LIST OF TABLES

Table 1. 1. Theoretical and actual efficiencies of different commercial fuel cells and their operating

temperatures [8]. ........................................................................................................................ 3

Table 1. 2. The effect of operating temperature of hydrogen fuel cell on the cell voltage and maximum

efficiency[72]. ......................................................................................................................... 11

Table 2. 1. The effect of water content in the polymerization solution to graft level. ................................ 30

xv

LIST OF EQUATION

Equation 1.1 ................................................................................................................................ 3

Equation 1.2 ................................................................................................................................ 7

Equation 1.3 ................................................................................................................................ 7

Equation 1.4 ................................................................................................................................ 7

Equation 1.5 ................................................................................................................................ 9

Equation 1.6 .............................................................................................................................. 10

Equation 1.7 .............................................................................................................................. 10

Equation 1.8 .............................................................................................................................. 10

Equation 1.9 .............................................................................................................................. 12

Equation 1.10 ............................................................................................................................ 12

Equation 1.11 ............................................................................................................................ 12

Equation 1.12 ............................................................................................................................ 12

Equation 1.13 ............................................................................................................................ 13

Equation 2.1 ............................................................................................................................. 27

Equation 2.2 ............................................................................................................................. 27

Equation 3.1 ............................................................................................................................. 41

Equation 3.2 ............................................................................................................................. 41

xvi

ABBREVIATIONS AND SYMBOLS

4-VP : 4-vinylpyridine

AAEM : Alkaline Anion-exchange Membrane

AFC : Alkaline Fuel Cell

AEMFC : Anion Exchange Membrane Fuel Cell

ATRP : Atom Transfer Radical Polymerization

BPMFC : Bipolar Membrane Fuel Cell

CHP : Combined Heat and Power

DCFC : Direct Carbon Fuel Cell

DFT : Density Functional Theory

DMFC : Direct Methanol Fuel Cell

DMSO : Dimethyl Sulfoxide

DVB : Divinylbenzene

ETFE : Poly(ethylene-co-tetrafluoroethylene)

FC : Fuel Cell

GL : Graft Level

IP : Immersion Precipitation

IPA : Isopropanol

MCFC : Molten Carbonate Fuel Cell

MFC : Microbial Fuel Cells

NMR : Nuclear Magnetic Resonance

OCV : Open Circuit Voltage

ORR : Oxygen Reduction Reaction

PA : Phosphoric Acid

xvii

PAFC : Phosphoric Acid Fuel Cell

PBI : Poly(benzimidazole)

PDI : Polydispersity Index

PEFC : Proton Exchange Fuel Cell

PEMFC : Polymer Electrolyte Membrane (Proton Exchange Membrane) Fuel Cell

PSSA : Poly(styrene sulfonate acid)

PSSS : Poly(sodium styrene sulfonate)

PVDF : Poly(vinylidene fluoride)

RH : Relative Humidity

RIGP : Radiation Induced Graft Polymerization

SOFC : Solid Oxide Fuel Cell

SPEEK : Sulfonated poly(ether ether ketone)

TAC : Triallyl-cyanurate

TFE : Tetrafluoroethylene

TGA : Thermogravimetric Analyses

THF : Tetrahydrofuran

TIPS : Thermally Induced Phase Separation

UTM : Universal Tensile Machine

VIPS : Vapor Induced Phase Separatio

1

1. INTRODUCTION

1.1. History of fuel cell

Luigi Galvani was the first person who discovered the field of electrochemistry. In his lecture on

the 30th of October 1786, at the Academy of Sciences of Bologna, he presented the result of his

study on animal electricity. By publishing Galvani’s results a controversy with Alessandro Volta

arose, who did not believe in animal electricity [1, 2]. The development of Volta’s battery which

was inspired by Nicholson and Bennet was another milestone in electrochemistry [3]. With the

help of Volta’s battery, Nicholson and Carlisle could electrolyze acid solution, and generate

hydrogen and oxygen [4]. The most important contribution to electrochemistry and

electromagnetism was obtained by Michael Faraday in his paper in 1821 that challenged the

previous theories on electromagnetism. In his later work he could establish the basis of today’s

electrochemistry [5].

Figure 1. 1. Schematic of Sir William Grove fuel cell 1839 [6].

The first fuel cell can be attributed to Sir William Grove. In 1839 he could demonstrate that it is

possible to generate electricity by passing the products of water electrolysis over the platinum

electrodes (Figure 1.1). Later on in 1889, Mond and Langer became the first researchers who

coined the term “Fuel Cell” (FC). They tried to scale up the Grove’s cell for electricity

production, but because of the poisoning of platinum catalyst by impurities in supporting gases,

2

and the high price of their FC, they could not scale up this technology. In the early 20th

century

there were some efforts made by Jacques to develop carbon batteries on the one hand, and fuel

cell mechanism by Bacon on the other hand. The first Proton Exchange Fuel Cell (PEFC) was

invented in 1955 by William Grubb at General Electric. Due to its low weight and compactness,

it was first deployed by U.S. Gemini program in 1962 [7]. Since 1960s, FC technology faced

many improvements and branched into different types of cells. Yet PEFC remained as one of the

promising types of FC due to its high efficiency.

1.2. Types of fuel cells

Since the invention of the first fuel cell by Sir William Grove different types of FC systems have

been developed, and they are categorized based on the fuel they use, the mechanism of ionic

transportation, the type of material being used in them and other criteria. The feature which all

the FC systems have in common is that they are all designed for energy conversion purposes.

Figure 1.2 shows the FC technologies already developed based on their operating temperature,

and Table 1.1 provides a detailed comparison between current FCs kinetics, theoretical

efficiency, practical efficiency and the range of their operating temperatures (Polymer

Electrolyte Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC), Alkaline Fuel

Cell (AFC), Phosphoric Acid Fuel Cell (PAFC), Solid Oxide Fuel Cell (SOFC), Molten

Carbonate Fuel Cell (MCFC), Direct Carbon Fuel Cell (DCFC)) [8].

Figure 1. 2. Classification of current commercial fuel cell technologies [8].

3

The most common commercial fuel cells have actual efficiencies ranging from 40% to 60% aside

from their operating temperature. Additionally, in the range of 80°C to 500°C only the PAFC

type of fuel cell with 40% electrical efficiency is operational. In a fuel cell, the rest of chemical

energy which doesn’t convert to electrical energy is released as waste heat to environment. This

heat energy can be partially utilized if consumed by a heat engine. According to Carnot

Efficiency, the maximum achievable thermal efficiency for any heat engine is a function of the

temperature of the heat source (Tmax) and the ambient’s temperature (Tmin) (Equation 1.1).

Therefore, high temperature fuel cells attracted more attention for Combined Heat and Power

(CHP) systems (Table 1.1), which enables these systems to benefit from a higher overall

efficiency [9, 10].

Table 1. 1. Theoretical and actual efficiencies of different commercial fuel cells and their operating temperatures [8].

Accordingly, it will be useful to discuss the current commercial and developing FC technologies

based on their operating temperature.

(1.1)

1.2.1. Low temperature fuel cells

This category encompasses the types of FC systems which normally operate below 100°C.

Among them, the application of Polymer Electrolyte Membrane, which is also known as Proton

Exchange Membrane (PEMFC) dominates the other ones. PEMFC can be divided into two

4

distinct categories: the first one is known as Proton Exchange FC (PEFC) for Hydrogen FC as

well as Direct Methanol FC (DMFC); and the second one is Anion Exchange Membrane

(AEMFC) systems in which the membrane is only permeable to hydroxide anions. Some other

technologies such as Bipolar Membrane (BPMFC) [11, 12], Laminar Flow Membrane-less FC

[13], and Microbial Fuel Cells (MFC) [14, 15], which can be categorized as low temperature fuel

cells, are still developing.

In case of PEFC, the membrane of FC assembly is only permeable to cations [16]. This feature is

typically achieved by introducing some acidic functionality to the polymer material of the

membrane. Therefore, once hydrogen, methane, or methanol is used as fuel only positively

charged hydrogen ions are able to pass through membrane and complete the reaction. In contrast,

in an AEMFC system, the membrane is only permeable to hydroxide ions which are negatively

charged [17, 18]. This property is achieved by introducing basic functionality to the membrane

which is also the reason why this type of membrane is also referred as Alkaline Anion-exchange

Membrane (AAEM). In general, polymers which either have acidic or basic property are called

Ionomers. Figure 1.3 shows the difference between PEFC and AEMFC systems.

Figure 1. 3. Difference between two types of systems for FC [18].

5

1.2.2. Intermediate temperature fuel cells

Normally intermediate temperature for fuel cell application refers to the temperature gap

between PEMFCs and SOFCs which is usually between 100°C to 600°C. This temperature range

has many advantages over low temperature for FCs such as: minimizing catalyst poisoning by

carbon monoxide (CO), reducing the requirement for noble metals as catalyst, improving the

efficiency, and solving the fuel cell flooding problem [19]. Originally, alkaline fuel cells used

alkaline metal hydroxides such as potassium hydroxide (KOH) for electrolyte. Thus, using

alkaline metal hydroxides enabled them to operate at temperatures above 100°C, ranking them

as intermediate temperature fuel cell [20]. In contrast, AFCs are very sensitive to the carbon

dioxide (CO2) contamination of electrolyte, which converts hydroxides to carbonates [21].

Another type of fuel cell which is able to work at an intermediate temperature range is

phosphoric acid FC [22]. In PAFC, phosphoric acid is used as an electrolyte material and it can

be retained with the help of an inorganic frame such as silicon carbide (SiC) [23]. In a very

similar fashion, PAFC can be used with organic frames in order to retain phosphoric acid, and

these types of membrane are usually referred to as high temperature (HT) PEMFC [24-28]. In

HT-PEMFC, besides phosphoric acid, other high temperature ionomers such as poly(vinyl

phosphonic Acid) is also used in the electrolyte structure to enhance the ionic conductivity at

high temperature [29-34].

The other attractive type of fuel cell is based on solid acids which also fit into this category.

Solid acids are monovalent or divalent metal cations combined with tetrahedral oxyanions [35]

having the general form of AxHy (XO4)(x+y)/2 (X is Se or P, S, As and A is NH4 or Cs, Ti, Li, K,

Rb) [36]. The size of alkali metal ions directly influences the melting point and proton

conductivity of the solid acids in such a way that as the size of cation increases the melting point

of the solid acid increases as well. Therefore, many of the studies focused on Cs+

hydrates as a

membrane capable of operating at temperatures as high as 250°C whether solely [37, 38], or in

combination with other inorganic/organic materials [39-43].

6

1.2.3. High temperature fuel cells

Solid Oxide FCs (SOFC) are another type of fuel cell that are named based on the material being

used as their electrolyte material, and are capable of operating between temperatures between

600°C and 1000°C. SOFCs, due to their high operating temperature, are one of the best

candidates for CHP systems [10, 44]. Additionally, they can benefit from using nickel and nickel

alloys as cheap substitute materials for the catalyst [45]. Although SOFC is a potential candidate

to dominate the market, because of technical constraints limited their application only to the labs

and prototypes. Some of these problems are: the extended start up duration, the use of expensive

materials for operation at elevated temperatures, and the failure of the system due to thermal

stress between the SOFC components (Figure 1.4) [46]. One approach to overcome these

problems is lowering the cell temperature around 600°C or lower, which is referred to as Low

Temperature (LT) SOFC. Some efforts are done by using Ceria-based materials such as CeO2,

CexZr1−xO2, and Ce1−xRxO2−x/2 (R: rare earth) and isovalent-cation–stabilized bismuth oxides [47,

48], yet these types of material suffer from electrical conductivity and sintering problems [44].

Perovskites are a category of ceramics which maintain suitable ionic conductivity for LT-SOFC

applications such as BaCe0.8Y0.2O3 and SrCe0.8Y0.2O3. The drawback of these ceramics is their

instability under water vapor and CO2 [49]. BaZr0.8Y0.2O3 is a perovskite proton conductor

material that has even higher conductivity than the aforementioned perovskites, and it is stable

under vapor and CO2 [50, 51]. La2Ce2O7 is another ceria based material which also exhibits

proton conduction for LT-SOFC applications [52].

Molten Carbonate Fuel Cell (MCFC) is a type of FC which uses molten alkali metal carbonates

such as LiCO3,NaCO3, KCO3 or a combination of them as electrolyte material, and has an

operating temperature of around 650°C, and is capable of using different types of fuel. This type

of fuel cell can benefit from using metal catalysts such as alloys of nickel including NiCr and

NiAl. One of the challenges of MCFCs is the corrosion of electrode material which limits their

operation life despite their high efficiency and low cost [53]. Direct carbon fuel cell (DCFC) is

another type of FC which operates at temperatures above 600°C, and it is the only FC that

operates on carbon as a solid fuel. The theoretical efficiency of this type of FC is 100% and due

to its high efficiency, it is considered a possible substitute for coal-burning power plants [54].

Most of DCFC designs are based on SOFC, MCFC, or molten hydroxide electrolytes [55, 56].

7

Figure 1. 4. Materials and related issues for SOFC [46].

1.3. Governing principles for proton exchange fuel cell

The process of oxidation and reduction in PEFC can be simplified through its half-cell reactions.

The most ideal case would be the chemical reactions of hydrogen and oxygen and their relevant

electron transfer. Equations 1.2 and 1.3, respectively, show the ionization reaction for H2 and O2

in a PEFC, and consequently after the electron transfer a water molecule forms (Equation 1.4).

H2 ↔ 2H+ + 2e

- (1.2)

½ O2 + 2e- ↔ O

-2 (1.3)

H2 + ½ O2 ↔ H2O (1.4)

8

Once hydrogen and oxygen are directly in contact, this electron transfer between the two atoms

causes combustion and heat generation in the reaction medium, and the generated electricity is

not utilizable. To utilize this electricity, as an alternative method of reaction, the two half-cell

reaction can occur on separate catalysts, and the ions will be able to combine to form the reaction

product later on. Norskov et al. performed a tremendous amount of research to better understand

the behavior of heterogeneous catalysts through density functional theory (DFT) and empirical

experiments [57-62]. The catalyst activity comprises two main processes: chemical adsorption

(chemisorption) and desorption. In the chemisorption, the molecule first binds with the s-band

orbital of catalyst and goes through hybridization. Depending on the electron filling of the

orbitals with respect to the Fermi level, the energy of bonding and anti-bonding can be

determined. In case of transition metals with d-band, the hybridization will also be affected by

the electron filling of these orbitals with respect to their Fermi level in such a way that as we

move from the left side of periodic table toward the right side, the chemisorption decreases and

the desorption increases. This trend leads to an optimum catalyst activity for different elements

for specific reaction and enthalpies. For example, in the oxygen reduction reaction (ORR) at low

temperature Pt is the closest element to the maximum catalyst activity, while at higher

temperature Ni depicts the highest activity for ORR [63].

Spillover is a process in which after a molecule undergoes decomposition on the surface of a

heterogeneous catalyst, the resulted products of catalyst activity go through a surface diffusion

process [64]. The diffusion process is mainly a function of surface properties, as well as ionic

concentration. Therefore, the diffusion of ionic species not only occurs on the catalyst itself, but

also on the catalyst supporting material. In the case of catalytic reactions, in which electron

donation and acceptance of reactants happen on the same surface, the reaction can continue until

the reactants are consumed. In contrast, in a FC catalytic reaction, since electron donation and

acceptance are occurring on separate catalysts, the limiting parameter for reaction is determined

by the formation of a double layer of generated ions with respect to the electrode potential. Since

the catalyst material and the reactants all have different Fermi levels, as they get in touch with

each other, they establish a potential difference which will lead to the generation of an electrical

field around the catalyst [65].

9

In order to complete the reaction, the generated ions on the surfaces of catalysts must be able to

reach each other to form the reaction product. Since these ions are still attached to the surface of

the catalyst by means of electrostatic forces, therefore no reaction will occur unless these ions

can be separated from the catalyst and travel through a medium. The process of separation of

ions from the surface occurs through electron exchange between two electrode catalysts. Due to

their opposite electrical charges, one electrode donates electrons (i.e. anode) and the other

electrode accepts electrons (i.e. cathode). In the case of water formation, the oxidation of

hydrogen molecule on anode releases two electrons. On the other hand, ORR on cathode requires

two electrons for each oxygen atom (equations 1.2 and 1.3). If there is no electrical potential

between two electrodes by connecting them electrically together, or applying an electrical load,

the electrostatic force between ions and catalyst surface will decrease, and ions are free to travel

to form water.

In order to utilize the electrical energy of the reaction the medium in which ions travel must be

electrically insulating while at the same time remaining ionically conductive. Ionic conduction

can occur in plasma [66], gases [67], supercritical fluids [68], liquids [69], and solids [70]. The

movement of ions in the medium is based on the principles of mass transfer. In general, the

modes of mass transfer contain:

1- Migration (drift): movement of particles in the medium by exerting electrical field.

2- Diffusion: movement of species due to the gradient of chemical potential in the medium.

3- Convection: movement of species by natural or forced convection of the medium (applies

only to fluids).

These phenomena in electrochemistry are abbreviated in the Nernst-Planck formula, the one-

dimensional form which is expressed by Equation 1.5, in which Ji is the flux of ions i in x

direction (mole/s.cm2), Di is the diffusion constant (cm

2/s), ( )/ is the concentration

difference, ( )/ is the potential difference, Zi is the charge (dimension-less), Ci is the

concentration (mole/cm3) of ions i, and v(x) is the speed (cm/s) with which a unit of volume in

solution moves along the x direction [71].

10

( ) ( )

( )

( ) (1.5)

From an overall point of view, the purpose of a fuel cell is converting the chemical energy of

reactants into electrical energy. Therefore, it is possible to define the theoretical efficiency of

fuel cells as the fraction of attainable electrical energy with respect to the chemical potential

energy of reactants (Equation 1.6).

(1.6)

In this equation is the maximum attainable efficiency, is the change of enthalpy (Joule)

for reactants and reaction products, and is the change in Gibbs free energy (Joule) which is

defined as follow (Equation 1.7):

(1.7)

In this formula T is the temperature (K°) and is the entropy (Joule/K°) or the irreversibility of

the system. Theoretically, the efficiency of a fuel cell will increase as the operating temperature

decreases. This is due to the fact that at a higher temperature more of the available enthalpy

converts to heat rather than to electrical energy. The efficiency of fuel cells is also directly

related to the change in Gibbs free energy through the Equation 1.8.

(1.8)

11

In this equation, n is the number of electrons involved in the formation of reaction products (for

equation 1.4, ), F is Faraday constant ( , e is the electron charge, NA is the

Avogadro’s number), and E is the cell potential or electromotive force (EMF), the calculated

values for hydrogen FC with respect to temperature are presented in Table 1.2 [72].

Table 1. 2. The effect of operating temperature of hydrogen fuel cell on the cell voltage and maximum efficiency[72].

Although it seems that at lower temperature the hydrogen fuel cell has more efficiency, in case

of methane (CH4) as fuel, the change in Gibbs free energy is less pronounced, leading to an

almost constant voltage at different operating temperatures. Some of the oxidation pathways of

methane are shown in Figure 1.5 [73].

Figure 1. 5. The effect of temperature vs. cell voltage E°(v) for hydrogen and methane as fuel [73] .

12

In a practical fuel cell, the thermodynamic efficiency of the cell is influenced by some additional

losses (overpotentials). The activation (or polarization) overpotential is the major loss in a fuel

cell which is the reduction of cell potential due to the rate of electron transfer of surface reactions

and can be explained by Tafel (Equations 1.9 and 1.10), or alternatively by Butler–Vollmer

formula (Equation 1.11).

(

) (1.9)

(1.10)

(

) (1.11)

is the difference between open circuit voltage (OCV) (V) and the voltage after current i

(A/m2) passes through the electrode, is the threshold current (A/m

2) which OCV starts to drop,

is the charge-transfer constant which relies on reactants and the selection of materials for

electrode, T is the temperature in Kelvin (K°), F is the Faraday coefficient, and R is the ideal gas

constant (Joule/K.mole). The activation overpotential can be reduced by increasing the working

temperature of FC, choosing correct materials for electrodes, maximizing the catalyst surface

area, and increasing the concentration of reactants or their pressure. The other loss in a fuel cell

is ohmic overpotential ( ), which is the resistance of ionic conductive medium toward

moving ions, and the electrical resistance in the electrical connections of the cell (Equation 1.12).

(1.12)

13

In this formula i is the current density (A/cm2) and r is the resistance per surface area (Ω/cm

2). In

order to minimize the ohmic loss, the ionic conductive medium should have a minimum

resistance, all electrical connections to the electrodes must be sufficiently conductive, and the

distance between two electrodes must be minimized.

( ) (1.13)

Another loss is the concentration (or mass transport) overpotential due to the reduction in

concentration of reactants which is directly related to the reduction of their partial pressure. This

phenomenon happens by blocking the supply of reactants which, in case of air, the presence of

nitrogen reduces the oxygen concentration at high currents, or in case of hydrogen fuel cell

concentration overpotential occurs by accumulation of water at electrode-gas interfaces. The

concentration overpotential can be calculated by means of Equation 1.13 in which m is typically

about ( ), and n about (m2/A) [72]. There are also cross-over and mixed

potential losses as a result of passing non-ionized species through the conductive medium,

electrical conductivity between two electrodes, and side reactions such as Pt-O bond formation in

case of hydrogen fuel cell [74].

The overall effect of these losses for an empirical fuel cell is shown in Figure 1.6, and it can be

observed that as the current-density elevates the fuel cell voltage drops in four distinctive

regimes.

Figure 1. 6. Typical performance of a hydrogen fuel cell and the effect of common overpotentials [73].

14

1.4. Proton exchange membranes

Proton exchange membranes, also known as polymer electrolyte membranes (PEM), are in

general referred to any ionic conductive electrolyte which conducts only positively charged ions,

which, in the case of a hydrogen ion, is called a proton. As already discussed, some ceramics

also exhibit proton conduction, but normally PEMFC is a low temperature FC which uses

polymer materials as electrolyte. Proton is the only ion which does not have any electron in its

orbitals, and as a result this cation has its own unique properties. It is believed that there are two

types of mechanisms involved in proton conduction: one is the vehicle mechanism in which a

proton in an aqueous system forms a hydronium ion (i.e. H3O), and the other one is the Grotthuss

or hopping mechanism in which protons hop from one molecule to the other one [75]. As a

proton hops from one oxygen site to another it faces a potential barrier which is a function of

distance. For infinitesimal distances, the electron orbitals of donor and acceptor hybridize to such

a value that the barrier completely disappears (Figure 1.7) [76].

Figure 1. 7. Semiempirical potential energy for proton transfer across hydrogen bindings of symmetrical conformations of the

type R-O-H...O-R for different oxygen distances Q and full stabilization of the surrounding [76].

The polymer electrolytes are usually non-homogeneous, and there are regions of non-conductive

and ionic-conductive channels [77, 78]. Therefore, the arrangement and connectivity of these

ionic channels also affect the proton conductivity of PEM. The hopping mechanism is not

limited to water, but also some acids such as phosphoric acid, which has the highest proton

conductivity among the other substances, also owes 98% of its proton conductivity to Grotthuss

mechanism [79, 80]. As a result, hydrated polymers with acidic groups are considered the most

suitable PEM structure. In contrast, by reducing the level of hydration the proton conductivity of

15

PEM also reduces up to the point that at 0% relative humidity the proton conductivity is almost

negligible [81]. For example, in case of poly(vinyl phosphonic acid) (PVPA) simulations suggest

that in comparison with phosphoric acid, the dominant mechanism for proton conductivity in

PVPA is a short distance vehicle mechanism of hydronium ion between the acid sites of the

polymer [82] (Figure 1.8).

Figure 1. 8. Grotthuss-type mechanism by means of hydronium ions in poly(phosphonic acid)[82].

For applications in which the operating temperature of FC is higher than 100°C the relative

humidity decreases, and maintaining the relative humidity at elevated temperatures requires

increasing the FC operating pressure. The proton conductivity at elevated temperatures can be

addressed by means of an acid-base type of salts. The Walden rule indicates that the ionic

conductivity is inversely proportional to the viscosity of the electrolyte [83]. Considering the

solid nature of an acid-base PEM, and according to the Walden rule it is expected that there is no

ionic conductivity for solid salts. In contrast, some researches have proven that the Walden rule

does not apply to polymeric systems [84, 85]. One of the HT-PEM systems which has been well

studied is the polybenzimidazole (PBI)-phosphoric acid (PA) system [27]. Experimental and

theoretical studies imply the fact that the proton conductivity in the case of acid-base membranes

relies on different mechanisms (Figure 1.9). These studies suggest that proton conduction

depends on the interactions between PBI-PBI, PA-PA, PBI-PA, PBI-water and PA-water [86-

89].

16

Figure 1. 9. Proton conductivity system for acid doped PBI-membrane [27].

1.4.1. Fluorinated proton exchange membranes

It is possible to use a variety of polymer electrolytes for the FC application. One branch of these

materials is based on fluorinated or partially fluorinated membranes. Instead of having a C-H

bond like hydrocarbons, these types of polymers have a C-F bonding in their structure, and

therefore they are also referred to as fluorocarbons or fluoroplastics. The substitution of

hydrogen with fluorine brings some changes in the physical and chemical properties of the

polymer due to the difference in their electronegativity and molecular weight. As an example,

polytetrafluoroethylene (PTFE) is a linear polymer similar to polyethylene (PE), but PTFE has a

higher melting point, a higher chemical resistivity, and a larger free volume due to its helical

molecular structure with respect to the zigzag structure of PE [90, 91].

During the past decade Nafion® produced by DuPont as the most successful PEM material has

been owing its superiority because of its fluorinated polymer structure. This material is

essentially a fluorocarbon with attached sulfonic acid groups. Being similar to Teflon®, Nafion

®

exhibits an excellent resistance to chemical attacks and an extremely low release rate of

degradation products into the surrounding medium. It also has a relatively high operation

temperature range, and may be used in many applications at temperatures up to 190 °C. Nafion®

has a high proton conductivity and acts as a superacid catalyst because its sulfonic acid groups

act as an extremely strong proton donor. The interaction of sulfonic acid groups with water

results in rapid water absorption and water transport through the Nafion® material [16].

17

Figure 1. 10. A) Nafion® B) Aquivion® C) 3M structures [92].

Even though Nafion®

has dominated the market, there are some drawbacks associated with

Nafion®, some of which include: high manufacturing cost, low conductivity for high temperature

applications and loss of chemical and mechanical stability at elevated temperatures. Besides

Nafion®, there are some other similar products offered by different companies such as Solvay

®

(Aquivion®) and 3M (Figure 1.10) [92]. Though all of these membranes have a very similar

polymeric structure, they have different synthesizing routes.

The process of synthesizing Nafion®

and Aquivion® starts with a tetrafluoroethylene(TFE)

monomer in gaseous form, and then it reacts with sulfurtrioxide (Figure 1.11) [93]. After the

completion of both pathways the result is a perfluoro(alkylvinyl ether) with a sulfonyl acid

fluoride monomer. The polymerization of this monomer with TFE will result in a brushed-like

fluorinated polymer with sulfonic acid groups [94]. In contrast, 3M developed a membrane

through electrochemical fluorination of a hydrocarbon materials(Figure 1.12) [95]. Since the

radical-attack on the acidic end-groups of the polymer backbone is the main cause of polymer

damage [96], the resulted polymer is further stabilized by post-synthesis fluorination.

18

Figure 1. 11. Different synthesis pathways for perfluoro(alkyl vinyl ether) with sulfonyl acid fluoride, above) DuPont method

for Nafion®, below) Solvay method for Aquivion [93].

Figure 1. 12. 3M method for synthesizing perfluoro(alkylvinyl ether) with sulfonyl acid fluoride monomer [95].

19

1.4.2. Sulfonic polymers and processes

The addition of acidic groups to monomers or polymers is an essential step in membrane

functionalization. Among the variety of acidic groups in hydrocarbons, sulfonic groups, which

are a result of sulfonating polymers and monomers, have been the center of attention for the

synthesis of non-fluorinated PEM systems. The sulfonic groups can be introduced by means of

the reaction of a sulfonating agent with hydroxyl or aromatic groups of hydrocarbons. The

earliest type of sulfonic polymer that was used in the Gemini program was a cross-linked

polystyrene sulfonic acid(PSSA) which had a short life due to oxidative stress [97]. Despite the

initial failure of PSSA membranes, the effort to improve the properties of aromatic polymer

membranes for applications below 60°C still continues.

The sulfonation of polymers, usually referred to as post-sulfonation, is usually done by

sulfonating agents such as: concentrated sulfuric acid, fuming sulfuric acid (oleum),

chlorosulfonic acid, acetyl sulfate, sulfur trioxide complexes and trimethylsilyl chlorosulfonate

((CH3)3SiSO3Cl). Some of the post-sulfonated aromatic polymers include: sulfonated styrene

copolymers [98, 99], sulfonated polyimides (SPIs) [100], sulfonated poly(phenylene)s [101],

sulfonated poly(arylene) types polymers [102] and sulfonated poly(phosphazene)s [103, 104].

Among the post-sulfonated aromatic polymers, sulfonated poly(ether ether ketone) (SPEEK) is

one of the most outstanding materials for membrane applications (Figure 1.13) [105]. In these

systems the degree of sulfonation can be controlled by changing the concentration of the

sulfonating agent, temperature or reaction time.

Figure 1. 13. Sulfonation of (PEEK) by sulfuric acid [106].

20

The alternative method used for synthesizing sulfonated aromatic membranes is polymerization

of monomers with a sulfonic group. Some of the possible sulfonated aromatic monomers for

PEM application include: sulfonated 4,4´-dichlorodiphenyl sulfone [107], 3,3´-disulfonated 4,4´-

difluorodiphenyl ketone [108], 3,3´-disulfonated 4,4´-dichlorodiphenyl sulfone [109, 110], 4,4´-

diamino-biphenyl 2,2´-disulphonic acid (BDSA) [100, 111-113]. The abovementioned

monomers are the ones suitable for condensation polymerization. Additionally, one may also use

a vinyl type of monomers such as sodium styrene sulfonate (SSS) in radical polymerization

applications.

1.4.3. Radiation induced graft polymerization

Grafting is a process in which a polymer is added to another polymer or substrate. There are two

methods of grafting, grafting-from and Ggrafting-onto systems. Both of these systems will lead

to brushed-like polymer structures. In the grafting-onto method, the functional group at the end

of a polymer reacts with a reactive site on a polymer with multiple reactive sites. On the other

hand, in the grafting-from method, polymerization occurs on a polymer backbone with multiple

active radical sites. These sites act as an initiator for radical polymerization, and therefore the

second polymer polymerizes from the backbone of the other polymer. Another option for

synthesizing brushed-like polymers is grafting-through by using a monomer which already

contains a long chain polymer attached to a vinyl group (Figure 1.14) [114].

Figure 1. 14. Methods for preparing comb-like polymers: (a) “Grafting-onto” (b) “Grafting-through” (c) “Grafting-from” [114].

21

Radiation induced graft polymerization (RIGP) is a grafting-from type of polymerization in

which active sites are generated by radiation. The received radiation dose is defined as the

energy passed through the material in Gray (Gy) or kilo Gray (kGy). The number of generated

active sites is proportional to the amount of radiation dose that the material received [115-132].

Figure 1. 15. Plausible mechanism of preparation of phosphoric acid doped poly(4-VP) grafted ETFE membrane [133].

The RIGP process can be performed as pre-irradiation and simultaneous-radiation

polymerization [127]. Since radiation passes through all the polymers, therefore active sites can

form on the polymer backbone whether it is a hydrocarbon or a fluorocarbon. This advantage

brings the opportunity to benefit from chemical, mechanical, and thermal stability of fluorinated

polymers, and at the same time using commercial vinyl-monomers in solution radical

polymerization system to develop partially fluorinated membranes. Fluorinated polymers such as

poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), poly(vinylidene fluoride) (PVDF),

poly(ethylene-co-tetrafluoroethylene) (ETFE) poly(tetrafluoroethylene-co-perfluoropropylvinyl

22

ether) (PFA) and poly(tetrafluoroethylene) (PTFE) are the most common substrate material for

fuel cell membrane applications [134]. Different combinations of polymers for low temperature

PEM applications such as: ETFE-g-Poly(trifluorostyrene) [130], ETFE-g-Poly(styrene sulfonic

acid) [124], ETFE- g-Poly(styrene sulfonic acid-co-acrylonitrile) [124], ETFE-g-Poly(α-

methylstyrene-co-acrylonitrile) [135] as well as for AAEM applications [136-138] have

developed with comparable ionic conductivity with respect to Nafion®. For HT-PEMFC

applications, RIGP provides a simple and applicable method to develop high temperature

membranes, and most of these studies are focused on acid-base type grafted fluorinated polymers

with vinyl-monomers containing amine groups such as vinylpyridine and vinylimidazole (Figure

1.15) [133].

23

2. NANO-STRUCTURED POLY(VINYIDENE FLUORIDE) GRAFT POLYSTYRENE

SULFUNIC ACID FOR PROTON EXCHANGE MEMBRANE

Research Objectives

The Fluorinated Nafion®

membranes produced by DuPont are the dominant type of membranes

used for Hydrogen fuel cell applications. These membranes are very expensive, and as a result it

is one of the obstacles for realizing the Hydrogen as an alternative source of energy. Therefore,

the main goal of this chapter is the development of an alternative proton exchange membrane for

fuel cell applications which expresses similar characteristics to Nafion®

. Styrene sulfonic acid is

the sulfonated styrene monomer which is in sodium salt form, and it can be easily polymerized to

obtain highly proton conductive polymers. Co-polymerization of poly(sodium styrene sulfonate)

from poly(vinylidene fluoride) powder (which is a fluorinated polymer) by using radiation graft

polymerization is considered to develop suitable proton conducting material for hydrogen fuel

cell for the first time. Additionally, a facile method used for the modification of nano-structure of

the membranes by means of vapour induced phase separation during casting process is

considered to modify the morphology of ionic-channels. After the casting process ends the

obtained membranes can be activated in an acidic aqueous solution and ready to be used in a

fuel cell.

24

2.1. Introduction

The role of fossil fuels in the immense technological development undertaken during the last

century is indisputable, and after almost a century, they are the main source of manmade energy

supply. Currently, as the energy demand is remarkably increasing, the main concern is targeting

the depletion of fossil fuels as well as its environmental. Among many developments in the field

of energy conversion, the proton exchange membrane fuel cell (PEMFC) is one of the most

promising candidates meeting many of the criteria as an alternative energy resource. However,

one main obstacles of the PEM fuel cell is the manufacturing cost of its fully fluorinated proton

exchange membrane that comprises 32% of the cost of PEMFC [139].

The first commercially available polymer electrolyte membrane (PEM), which still dominates

the market, is Nafion® that is a perfluorosulfonic acid membrane. However, perfluorosulfonic

acid membranes have a high manufacturing cost due to their complex fluorine chemistry. A

variety of polymer formations have been proposed as an alternative for perfluorosulfonic acid

membranes including: sulfonated poly(styrene), sulfonated poly(imide), poly(phosphazene),

poly(benzimidazole), poly(arylene ether), poly(sulfone), poly(sulfoneether) and

poly(phenylsulfone) [16]. Radiation induced graft polymerized sulfonic acid membranes are one

of the best alternatives to Nafion® due to the advantages of their preparation method, ease of

control over tailoring the membrane properties, as well as their low cost [115-118, 120, 122-

127, 129, 130].

In radiation induced graft co-polymerization, the polymer film can be radiated by means of high

energy electron beam or gamma ray. As a result of radiation, active radicals on the polymer

backbone are formed. Therefore, the copolymerization process can be initiated from these

radicals [122, 140]. Traditionally, styrene is incorporated to fluorinated polymers by radiation-

induced grafting, and later the graft copolymer is sulfonated by means of a sulfonating agent

[141]. This method for PEM preparation is known as the two-step radiation induced graft

copolymerization in literature.

Radiation grafted sulfonic acid membranes are usually prepared by the radiation-induced

grafting of the styrene monomer onto the partially fluorinated polymer films (such as ETFE,

FEP, PVDF). Partially fluorinated polymers are also known for their high mechanical, thermal

25

and chemical resistivity. Among fluorine based polymers, poly(vinylidene fluoride) (PVDF)

exhibits high mechanical strength, good chemical resistance and thermal stability as well as

aging resistance to withstand to fuel cell conditions. Moreover, PVDF demonstrates good

processability, and it is also soluble in common solvents [142]. PVDF-based micro-porous

membranes are usually prepared by means of the controlled phase separation of polymer

solutions into two phases. This transformation can be accomplished in several different ways,

namely: (a) thermally induced phase separation (TIPS); (b) controlled evaporation of solvent

from three component systems; (c) vapour induced phase separation; and (d) immersion

precipitation (IP) [143]. Li et al. investigated the effect of water vapour on PVDF-

dimethylformamide (DMF) solution system for relative humidity(RH) ranging from 0% to 60%

at room temperature [144]. In their report they concluded that at 60%RH the PVDF goes through

a phase separation in DMF and forms particles in micron and sub-micron sizes.

Lehtinen et al. and Slade et al. studied the graft copolymerization of styrene from the PVDF film,

and later Lu et al. studied PVDF powder instead of PVDF film via the two-step radiation induced

graft copolymerization [145-147]. However, one main disadvantage of the two-step radiation

induced graft copolymerization is that the high degree of sulfonation cannot be achieved without

damaging the grafted membranes due to the strong sulfonating agent/solvent media. Instead

sulfonated monomers, such as sodium styrene sulfonate (SSS), can directly introduce the pentant

sulfonic acid groups to the graft copolymer. Kim et al. used the direct grafting of SSS on the

PVDF powder through the atom transfer radical polymerization (ATRP), while Su et al. applied

the same system through redox initiation [148, 149], and finally radiation induced graft

polymerization was used by Kim et al. and Nasef et al. [150, 151]. This method, which is also

referred to as the single-step graft polymerization, has advantages in terms of simplification of

synthesis process, increase of sulfonation efficiency, as well as reduction of production cost in

comparison to the two-step graft polymerization method[141].

Previously, Nasef et al. studied the effect of pH and various type of acids over polymerization

kinetics, and it was demonstrated that the pH of solvent system has a drastic effect on the

polymerization level [152]. Additionally, in their study, it has been shown that sulfonic acid led

to higher graft levels compared to other acids (such as HCl, HNO3, CH3COOH).

26

In this study the radiation-induced graft polymerization of SSS to PVDF powder was studied.

For the first time in literature, the powder form of PVDF was chosen in order to increase

monomer diffusion through the polymer backbone. An aqueous dimethyl sulfoxide (DMSO)

solution, as a less hazardous system, was suggested, and its kinetics has been studied by means

of NMR spectroscopy. The resulting powder was dissolved in DMSO [149], and cast as a thin

film by means of the tape casting method in order to have a high quality and homogenous and

dimensionally stable cast membranes, and was further modified by VIPS method to form a high

porosity membrane. The PVDF-g-PSSA proton exchange membranes were studied in details for

fuel cell related properties including ex-situ proton conductivity, water up-take, mechanical and

thermal properties in comparison to Nafion®

NR-211.

2.2. Experimental

2.2.1. Material

High molecular PVDF powder (Mw 380,000) was obtained from Solef, and Sodium 4-

vinylbenzenesulfonate (90%), DMSO (99.5%), H2SO4 (97%), HCl(38%), Methanol (99.9%)

were all purchased from Sigma Aldrich. All the materials were reagent grade and used as they

were received without any further purification. De-ionized water with 18MΩ resistivity was used

during the synthesis and conditioning of graft copolymers during the study.

2.2.2. Radiation induced graft copolymerization

The PVDF powder was weighted, and packed in small polyethylene plastic bags. The irradiation

process was performed in γ-rays via 60

Co source at 50 kGy total irradiation dose and at room

temperature. After irradiation, the PVDF polymer was kept in deep freeze. The polymerization

performed in the 1.5 mole/L SSS aqueous solution of DMSO with PVDF/SSS w:w ratio of 1:3,

water/DMSO v:v ratio of 1:4, and sulfuric acid concentration of 0.2 mol/L. The prepared

solution was degassed with N2 for 30 minutes, and was then left at 60°C in different reaction

time. After the grafting process, the resulted polymer was precipitated with acetone first and then

27

further precipitated with methanol, washed with water and filtered, and dried in oven for 24

hours at 60°C.

2.2.3. Membrane preparation

The obtained graft copolymers with different graft levels were dissolved in DMSO with 15%

wt.% ratio at 105°C, and were then degassed in vacuum and cast over a glass plate by means of

the tape casting method. In order to obtain nano-structured morphology in the membranes, they

were exposed to 60%RH in air atmosphere until the cast solution became opaque through VIPS

process. Later on, the samples were left in the vacuum oven at 180°C until they were shaped into

a 40μm thick thin film. It should be noted that the temperature was considered in such a way as

to be higher than the melting point of PVDF, but below the boiling point of DMSO in order to

achieve better mechanical properties similar to melt casting membranes. The resulted films were

activated in 1 M hydrochloric acid at 60°C for 12 hours, and then washed with deionized water

for several times prior to use.

2.2.4. Characterization of membranes

The 1H-NMR (VARIAN INOVA AS500) was used to determine the number of hydrogen atoms

in the phenyl group of PSSS in comparison to the number of hydrogen atoms in PVDF by means

of measured molar ratios. The graft level of polymer was calculated through Equation 2.1:

(2.1)

where Wg and Wo are the weights of grafted and pristine PVDF powder, respectively. The water

up-take of membranes was measured by comparing the weight of fully humidified membranes

with respect to their dry weight. After activating the membranes, they were soaked in water for

24 hours, and later on the extra water on the membranes was wiped with tissue paper, while their

weight was measured immediately. The water up-take was calculated through Equation 2.2:

28

(2.2)

where Wh and Wd are the weights of humidified and dry membranes, respectively. Additionally,

the proton conductivity of the membranes was tested in the Becktech 4-point probe conductivity

device at room temperature and 100% humidity. Thermogravimetric analyses (TGA) of the

samples were performed by Shimadzu DTG-60H, in comparison with Nafion® NR-211, under

the nitrogen atmosphere with the increment of 10 °C/min. The tensile strength of the membranes

was measured by using the Universal Tensile Machine (UTM) (Zwick/Roell Z100) at 100%

humidity and room temperature in comparison to Nafion® NR-211 by leaving the UTM samples

in water, and performing the test right after. Finally, a membrane with the highest ionic

conductivity was selected for fuel cell performance, and it was stacked with commercial Pt

electrodes with 0.5 mg/cm2 loading. Current-scan test was performed by Scribner 850e fuel cell

test bench at 60°C and 80%RH.

2.3. Results and discussion

The copolymerization of SSS monomer from PVDF powder was performed through a single-step

radiation induced graft copolymerization. Different solution systems based on water and alcohol

mixture were studied and no significant grafting could be observed. The aqueous DMSO solution

was considered a suitable candidate for polymerization, since DMSO is totally water miscible,

and also PVDF and SSS are both miscible in DMSO. Therefore, by adding water to DMSO, it is

possible to control the solvent up-take by PVDF powder in such a way that SSS monomer could

gain access to the active sites of PVDF polymer. Additionally, in literature it was shown that the

presence of water in the polymerization solution plays a positive role in the graft level both for

hydrophobic and hydrophilic monomers [153]. Environmental concerns and health hazards of

other similar solvents such as DMF also motivated us to consider DMSO as the key solvent for

polymerization.

Although the solution did not totally dissolve the PVDF polymer, during the filtration and

washing of PVDF-g-PSSS, it was observed that a precipitation step is required to prevent

coagulation of grafted polymer by means of phase separation. Consequently, acetone was

introduced to the solution as a spacer and anti-solvent for SSS monomer and homopolymer in the

29

solution. Later on, methanol was introduced to the solution to precipitate PVDF-g-PSSS, and

finally the resulted system was washed in 60°C water to remove solvents, homopolymers and

unreacted monomers.

In the membrane casting process, PVDF-g-PSSS copolymer was dissolved in DMSO with 15%

wt. ratio at 105°C in order to remove the absorbed water. The copolymer solution was cast on

glass plate by means of the tape casting technique. Due to the high boiling point of DMSO, the

PVDF-g-PSSS and DMSO solution is intrinsically capable of absorbing water vapour from the

environment to induce phase separation of polymer. This behavior led to benefiting from VIPS

to form a highly porous structure of cast membranes by leaving the samples after tape casting in

60%RH air until they formed an opaque appearance. During the film preparation it was

observed that the mechanical properties of the films highly depended on the solvent evaporation

temperature that is very similar to the pristine PVDF casting systems. To the best of our

knowledge, a new solution casting system was developed in such a way that the evaporation

temperature of the solution was kept above the melting point of PVDF but below the boiling

point of DMSO. In this method the resulted films benefited from the mechanical properties of the

melt casting membranes. The resulted film was separated by using a water and methanol solution

since the water itself can be absorbed too much by the membrane and causing stress and

deformation in the membrane. Later on, the membrane was kept for 24 hours at 60°C to remove

the remained solvents, and activated in 1M HCl aqueous solution for 12 hours. The PVDF-g-

PSSA membranes were washed several times before drying.

30

10 8 6 4 2 0

ca

ppm

a

b

S

O

O

ONa

F

F

F

F

b c

b c

Figure 2. 1. 1H-NMR result of PVDF-g-PSSS, “a” peaks belong to PVDF, and “b” and “c” peaks belong to SSS.

2.3.1. Graft level

The covalent bonding between PVDF and PSSS was verified by 1H-NMR at peaks 6.5 and 7.5

ppm (Figure 2.1) [154][155]. By knowing the molar ratio of SSS monomer, the graft level was

calculated with respect to PVDF. Grafting reactions with different combinations of water content

and 0.2M H2SO4 concentration were performed in order to determine the optimum graft

condition. It was noticed that the maximum graft level occurs at 20% v% water in the solution

(Table 2.1). Moreover, it was observed that H2SO4 prevents the polymer damage by reducing the

pH level of the copolymerization solution.

Table 2. 1. The effect of water content in the polymerization solution to graft level.

Water Content (v/v%) 10% 20% 30% 40% 50%

Graft Level (wt%) 30% 35% 30% 25% 4%

31

By varying the grafting time, it was indicated that grafting occurs very fast at the beginning of

the reaction, but further increase in the experiment duration does not have a significant effect on

the graft level (Figure 2.2). It was demonstrated that grafting continues up to one hour and after

one hour the grafting process stops at around 35% wt%. This behavior can occur due to the

solution polarity and solutes concentration, or radical transfer to the homopolymers which is very

common in the other irradiation grafting systems [126, 156, 157]. In addition, some other

behavior might be associated with the grafting behavior of SSS due to forming an amphiphilic

polymer structure such as the gel effect [158].

0 5 10 15 20 25

10

15

20

25

30

35

40

45

50

0.0 0.2 0.4 0.6 0.8 1.0

10

15

20

25

30

35

Gra

ft L

evel (%

)

Time (hours)

Time (hours)

Gra

ft L

evel (%

)

Figure 2. 2. Graft level of PVDF-g-PSSS with respect to reaction time.

2.3.2. Water up-take

The membranes prepared using the VIPS method with varying graft levels after the casting

process were activated, dried and hydrated again in order to measure their water up-take (Figure

2.3). The membranes with graft levels below 18% did not show any significant water up-take. As

32

the graft level increases, there is a sharp increase in water up-take up to 50%. Later on, although

the graft level increases, the water up-take increases at a much slower rate. This behavior can be

contributed to the interconnection of nano-voids inside the membrane which become less

pronounced as the graft level increases.

15 20 25 30 35

10

20

30

40

50

60

70

80

90

100W

ate

r up-t

ake (

%)

Graft Level (%)

Figure 2. 3. The relation between graft level and water up-take of membranes prepared by VIPS method.

2.3.3. Proton conductivity

Comparing the proton conductivity of PVDF-g-PSSA membranes at different graft levels reveals

that there is not a considerable conductivity below 18% grafting degree (Figure 2.4). Similar to

Nafion®, as the amount of water increases, the conductivity is enhanced, too. The water channels

and clusters grow and the amount of available free liquid water rises, as well. This results in an

enhancement of Grotthuss-type hopping, which increases the proton transport [16]. In order to

have a high conductivity, these water clusters must be interconnected to facilitate the Grotthuss-

type conductivity. In order to shape the water channels, the interconnection of water clusters

occurs at graft levels above 16%, and at room temperature for the current system.

The scanning electron microscopy of membranes revealed the formation of interconnected nano-

spheres of grafted polymer (Figure 2.5). Since the graft polymerization is not a controlled

33

polymerization, therefore it is expected that the graft polymerization has a wide polydispersity

index (PDI). As a result, the formation of nano-spheres can be explained in such a way that the

penetration of water vapour in polymer solution system causes the less grafted PVDF branches

(which are more hydrophobic) to first form the core of particles roughly around 200 nm, and the

more grafted polymer backbones (which are less hydrophobic) to form the shell and the

interconnections between particles.

15 20 25 30 35

-10

0

10

20

30

40

50

60

70

80

90

100P

roto

n C

on

du

ctivity (

mS

/cm

2)

Graft Level (%)

Figure 2. 4. Proton conductivity of the membranes prepared by tape casting and mold casting.

This phenomenon results in the formation of ionic channels and their efficient interconnections.

The formation of ionic domains through modifying the architecture of water channels is the

reason for the increase in ionic conduction [159].

Figure 2. 5. Scanning electron microscopy imaging of sub-micron structure of PVDF-g-PSSS membrane with 35% graft level.

34

2.3.4. Thermogravimetric analysis

Thermal analysis of the membranes was performed up to 800°C with the rate of 10 °C/min to

evaluate the membrane properties (Figure 2.6), and these results are very similar to PSSS and

PSSA thermogravimetric results [160]. Up to 100°C PVDF-g-PSSS, and its acidic form (PVDF-

g-PSSA), and Nafion®

NR-211 all show a significant loss which is mostly due to the evaporation

of water in their hydrophilic structure. From 100°C to 250°C all the membranes share an

insignificant weight loss, but above 250°C the rate of degradation starts to increase with the

slowest rate belonging to PVDF-g-PSSS. Since during membrane preparation, the activation of

membranes comes after film casting procedure, therefore PVDF-g-PSSS is still in salt form, and

it has the least susceptibility toward thermal degradation during the evaporation of solvent at

180°C.

0 200 400 600 800

0

20

40

60

80

100

Mass (

%)

Temperature (C)

PVDF-g-PSSA

PVDF-g-PSSS

NR-211

PVDF

Figure 2. 6. Thermogeravimetric analysis of PVDF-g-PSSA compared to pristine PVDF, PVDF-g-PSSS and Nafion® NR-211.

2.3.5. Mechanical properties

The tensile strength test was performed for the determination of the mechanical properties of

membranes at 100%RH (Figure 2.7). As it was observed, the PDVF-g-PSSA radiation grafted

membranes at 100%RH exhibit a higher plasticity than Nafion®

NR-211 at 30% graft level. In

the graph, as the graft level increases the mechanical properties reduce up to 20% graft level.

35

Due to a higher water uptake, membranes with a 20% more graft level show more elongation.

The increment in grafting degree after 20% slowly reduces membrane stiffness and increases its

elongation. In addition, we observed that the presence of water in the casting solution (during

VIPS process) affects both mechanical properties and membrane porosity with respect to solvent

evaporation temperature. The membranes which were prepared below 100°C essentially have

very weak mechanical properties, but posssess instead a very high proton conductivity up to 200

(mS/cm2). As the temperature rises, the phase separation becomes less pronounced, and the

polymer chains become orderly and oriented, resulting in higher mechanical properties and less

proton conductivity up to the melting point of PVDF.

0 10 20 30 40 50

0

10

20

30

40

50

Str

ess (

MP

a)

Strain (%)

PVDF

10% Graft

20% Graft

30% Graft

NR-211

Figure 2. 7. The universal tensile stress results of fully humidified PVDF-g-PSSA membrane with different graft levels.

2.3.6. Fuel Cell performance

The fuel cell performance of 35% grafted PVDF-g-PSSA membrane was measured at 60° and

80%RH under H2/O2 feed gases (Figure 2.8). The commercial electrodes were used without any

further modification. The cell demonstrated an open circuit voltage (OCV) of 0.97(V), then after

applying the load it was reduced to 0.8(V) due to activation losses. Interestingly, the current-

voltage curves followed almost a straight line up to 0.01(V) which indicates the absence of

diffusion losses at high current densities at three phase boundary. The maximum power density

36

of 250 (mW/cm2) was achieved at 0.4(V) and 650 (mA/cm

2). Although the PVDF-g-PSSA

membrane was prepared for the first time, compared to Nafion® NR-211 the prepared membrane

could show relatively good results. Also like any other styrene sulfonic acid based membrane,

the PVDF-g-PSSA membrane also demonstrated reduction in performance during continuous

operation which is due to losing the sulfonic acid groups by means of hydroxyl radicals [140].

0 500 1000 1500 2000

0.0

0.2

0.4

0.6

0.8

1.0

PVDF-g-PSSA

PVDF-g-PSSA

Nafion NR-211

Nafion NR-211

Cell Current (mA/cm^2)

Vo

lta

ge

(V

)

0

100

200

300

400

500

600

700

800

Po

we

r (m

W/c

m^2

)

Figure 2. 8. Current-voltage and current-power of fuel cell performance of 35% grafted PVDF-g-PSSA at 60°C and 80%RH vs.

Nafion® NR-211 at 80°C and 60%RH.

2.4. Conclusions

A facile method to prepare a sulfonated proton exchange membrane with nano-porosity was

successfully developed and characterized for the first time in the literature. The grafting process

of PVDF-g-PSSS was verified by means of H-NMR, and further membrane characterization was

carried out for graft level, ionic conductivity, thermogravimetric analysis, and measuring

mechanical tensile strength. By means of the VIPS method, the developed amphiphilic grafted

polymer membranes demonstrated conductivities comparable to Nafion®. Additionally, a new

solution casting method was used to retain the mechanical properties of membranes as close to

the melt casting process as possible. In the actual fuel cell environment, the PVDF-g-PSSA

membranes could present fuel cell characteristics relatively close to Nafion®

NR-211.

37

3. CROSS-LINKED PROTON EXCHANGE MEMBRANES BY RADIATION INDUCED

GRAFTING OF 4-VINYLPYRIDINE AND DIVINYLBENZENE FROM

POLY(ETHYLENE-CO-TETRAFLUOROETHYLENE) FILMS

Research Objectives

The classical hydrogen fuel cell needs to operate bellow 80°C because of the degradation of

Nafion®

membrane above this temperature. Low temperature operation of fuel cell results in CO

poisoning of electrodes, low energy density, and water formation in the gas manifolds.

Therefore, in this project an acid-base type of membrane for fuel cell applications between 80°C

to 120°C by means of radiation graft polymerization of cross-linked poly(4-vinylpyridine-co-

divinylbenzene) from poly(ethylene tetrafluoroethylene) films with consequence phosphoric acid

doping is proposed for the first time. Since during graft polymerization process the mechanical

properties of films usually decrease, the introduction of cross-linker (divinylbenzene) in the

grafted polymer matrix is expected to improve the mechanical properties of grafted membranes.

Additionally, in this study the influence of cross-linking over polymerization kinetics, mechanical

properties, acid up-take, proton conductivity and fuel cell performance will also be studied.

38

3.1. Introduction

Energy supply plays a crucial role in economic, environmental and social development. A

sustainable society requires energy resources which are economically affordable,

environmentally friendly, and as little as possible relying on depletable or foreign resources. Fuel

cell technology, among other types of renewable energy resources, has already proven its

capabilities. Although this technology is utilized in many different sectors, yet there are

possibilities to improve performance and efficiency of fuel cell systems. One of the promising

areas of research in this field is high temperature polymer electrolyte membrane fuel cells (HT-

PEMFC) operating at temperatures above 100°C. HT-PEMFC is very desirable due to improved

electrode kinetics, enhanced ionic conductivity and reduced humidification. The performance of

low temperature (LT) PEMFC normally suffers from catalyst poisoning by carbon monoxide

[161], water flooding problem in the cell [162], and poor performance for combined heat and

power (CHP) systems. Therefore, increasing the operating temperature of PEMFC has recently

attracted the attention of researchers [26].

Polymeric proton exchange membranes (PEM), due to their acidic groups that are attached to

their polymer backbone, are capable of proton conduction through nono-sized water channels.

Membranes with perfluorinated sulfonic acids such as Nafion®

manufactured by DuPont are the

most widely used PEM for fuel cell applications due to their high proton conductivity and

stability under harsh conditions. For operating temperatures above 100°C, the relative humidity

of PEM decreases and this leads to a reduction or loss of proton conductivity in the PEM [163,

164]. In addition, at elevated temperatures the membrane degradation is facilitated, which

consequently results in the loss of ionic conductivity [165].

39

Phosphoric acid doped polymer membranes are considered a suitable candidate for PEM

applications. In this method a polymer membrane with basic pendant groups such as

poly(benzimidazole) (PBI) is soaked in phosphoric acid [166]. The attraction between

phosphoric acid and the basic sites of the membrane results in the immobilization of phosphoric

acid in the polymer matrix. Besides PBI, other types of basic polymers such as sulfopropylated

poly(benzimidazole) [167], polybenzimidazole [168], phosphonated fully aromatic polyethers

[169], sulfonated polybenzimidazoles [170] were also developed for HT-PEMFC.

Radiation induced graft polymerization (RIGP) is a grafting-from type of polymerization in

which active sites are generated by radiation. Recently, in order to provide a high acid doped

acid-base polymer membrane system RIGP was used to graft polymerize basic monomers from

poly(ethene-co-tetrafluoroethene) (ETFE) films. The grafting of 4-vinylpyridine (4VP) from

ETFE films has so far provided excellent fuel cell performance for HT-PEMFC applications

[132, 133, 171-173]. During the radiation and grafting process the ETFE film partially loses its

mechanical strength [117]. Therefore, Nasef et al. studied the effect of triallyl-cyanurate (TAC)

on ETFE-g-P4VP, and they observed an improvement in the mechanical properties of cross-

linked membranes [174]. Additionally, they observed a higher phosphoric acid up-take during

the doping process which, in turn, led to a higher ionic conductivity of grafted membranes. In

another study Chen et al. studied the effect of different cross-linkers on the properties of ETFE

membranes, and he concluded that divinylbenzene (DVB) shows better chemical stability

compared to the other cases in their study [175].

In this work, we studied the effect of divinylbenzene (DVB) cross-linker on the physicochemical

properties of irradiation graft polymerization of 4-vinylpyridine and poly(ethylene-co-

tetrafluoroethylene) (ETFE) films for HT-PEMFC application. The process commences with

cutting ETFE films in desired dimensions, and later on irradiating them with gamma rays by

using a suitable dosage to generate radicals on the polymer backbone. Afterwards, graft

polymerization proceeds through an oxygen free solution of monomer(s) and suitable solvent(s)

by immersing the irradiated film. The resulted film from polymerization is washed, dried and

doped with phosphoric acid.

40

Figure 3. 1. Mechanism of preparation of cross-linked phosphoric acid doped poly(4VP) grafted ETFE membrane.

The conjugation of phosphoric acid with nitrogen in a pyridine ring provides a suitable condition

for proton conduction at elevated temperatures, while ETFE backbone provides the required

mechanical stability for the membrane. In order to further improve the mechanical properties of

membrane, DVB is introduced during the polymerization to strengthen its mechanical properties

due to the cross-linked rigid network (Figure 3.1).

3.2. Experimental

3.2.1. Materials

The base polymer ETFE with 25 micron thickness was purchased (Nowoflon ET-6235) from

Nowofol GmbH (Siegsdorf, Germany). The reagents isopropanol (IPA), tetrahydrofuran (THF),

4-vinylpyridine (4VP), divinylbenzene (DVB), phosphoric acid (PA) were all purchased from

Sigma Aldrich, and were used without any further purification.

H2C

H2C

F2C

F2C

ETFE base polymer

γ-ray irradiation

Activated polymer

DVB (crosslinker)

N

4-VP

grafting

Grafted film

Acid doping

Proton exchange membrane

H2CHC

NH

n

P

OHO

HOO

H2CHC

N

41

3.2.2. Membrane preparation

The ETFE films were first cut and then washed with ethanol, and later on dried overnight. The

films were afterwards weighted, and packed in small polyethylene plastic bags. The irradiation

process was carried out in γ-rays via 60

Co source at 50 kGy total irradiation dose at room

temperature. After irradiation, the ETFE films were kept in deep freeze. The polymerization

performed in a 36 (ml) polymerization solution of IPA, THF and 4VP with volumetric ration of

1:2:3 in a cylindrical reactor. The DVB was added to the solution with different volumetric ratios

with respect to 4-VP concentration ranging from 0% to 2%. The prepared solution was degassed

with nitrogen for 30 minutes, and then left at 60°C in different reaction times. After grafting the

process, the resulted polymer was washed in a 1:1 volumetric solution of IPA and THF

overnight, and dried in the oven for 24 hours at 60°C. The grafted ETFE-g-PVP membranes

were later on acid doped by soaking the membranes in PA with 85% concentration for 20 hours.

After acid doping, the excess acid was wiped from the surface of membranes with tissue paper

and without any washing.

3.2.3. Characterization of membranes

To measure the graft level, the radiation grafted copolymers were left in the room conditions

after drying in the oven in order to lose their electrostatic charge during drying. Later, the graft

level was measured by comparing the weight increase of the membranes with respect to their

original weight. The equation for graft level is as follows (Equation 3.1):

(3.1)

where Wg and Wo are the weights of grafted and original ETFE films, respectively. The

phosphoric acid up-take of membranes was measured by comparing the weight of fully acid

doped membranes right after the doping process with respect to their grafted weight. The PA up-

take was calculated by (Equation 3.2):

42

(3.2)

where Wd and Wg are the weights of acid doped and grafted membranes, respectively. In

addition, the proton conductivity of these membranes was measured at different temperatures

and relative humidities under N2 by means of Becktech 4-point probe conductivity device and

Scribner 850e fuel cell test station. The tensile strength of the membranes was measured by

Universal Tensile Machine (UTM) (Zwick/Roell Z100) at 60%RH and room temperature. The

effect of DVB on the surface properties of grafted membranes was also investigated by

measuring the contact angle. Scanning electron microscopy (SEM) imaging and energy-

dispersive X-ray spectroscopy (XEDS) mapping of membranes were performed by Zeiss Gemini

electron microscope. Additionally, membranes with DVB and without DVB were selected for

fuel cell performance, and they were stacked with commercial PTFE treated electrodes with 0.5

mg/cm2

Pt loading. Finally, the current-scan test was carried out by Scribner 850e fuel cell test

bench at 50%RH and varying temperatures.

3.3. Results and discussion

3.3.1. The effect of reaction time on graft level

The polymerization reaction was performed at 60°C by immersing the cylindrical reactor in a

silicon oil bath after placing the radiated film in the solution and purging nitrogen. Different

reactions by varying reaction time for 0% DVB concentration and 1% DVB concentration in

50% monomer concentration solution of 1:2 IPA-THF solution were also performed. After

washing grafted films with IPA-THF solution overnight, the membranes were dried and

weighted. The graft levels for the obtained membranes for durations from 1 to 6 hours are plotted

in (Figure 3.2). Quite similar to the other free radical graft polymerizations, up to one hour the

reaction rate is very fast, but as the reaction proceeds the rate of reaction slows down [116, 118,

123]. The reason for this phenomenon is the radical transfer from grafting sites to the solution.

This effect is very perceivable especially in the presence of DVB in such a way that after 6 hours

the polymerization solution totally turns into a gel. The main reason for considering the IPA-

43

THF binary solution was due to the fact that this system is capable of forming a transparent

mixture of polymerization elements without any phase separation.

0 1 2 3 4 5 6

0

10

20

30

40

50

60

70

80

90

100

Gra

ft level (%

)

Reaction time (hours)

0% DVB

1% DVB

Figure 3. 2. The graft level of ETFE-g-PVP films with 0%DVB and 1%DVB at 60°C, 50 kGy and varying reaction time.

3.3.2. The Effect of DVB concentration on graft level

0.0 0.5 1.0 1.5 2.0

30

35

40

45

50

55

60

65

70

75

80

Gra

ft leve

l (%

)

DVB concentration (%)

Figure 3. 3. The effect of DVB concentration on graft level of 50 kGy ETFE films at 60°C and 4 hours grafting time.

Comparing the effect of DVB on graft level of membranes, it is evident that DVB addition to the

monomer could increase the graft level by more than 40% (Figure 3.3). This effect can be

44

explained by fixation of growing chains and preventing the radical termination [118, 175]. The

graft level increases considerably for 0.5% and 1% samples, and for the samples with higher

DVB concentration the increase is negligible.

3.3.3. The effect of DVB on phosphoric acid up-take

0.0 0.5 1.0 1.5 2.0

90

100

110

120

130

140

150

Acid

up-t

ake

(%

)

DVB concentration (%)

Figure 3. 4. The effect of DVB concentration on the phosphoric acid up-take during doping process.

Phosphoric acid doping was performed by immersing the grafted films in 85% phosphoric acid

for 20 hours. Later the extra acid of the membranes was wiped with tissue paper, and

immediately weighted. The results of acid doping indicate that as the graft level increases, the

acid up-take also increases. In contrast, introducing the cross-linker has a negative effect on the

acid up-take by limiting the polymer matrix from expansion (Figure 3.4). Increasing the DVB

amount up to 1% maximizes acid up-take, and the further increase in DVB results in lower acid

up-take despite the fact that the graft level increases.

45

3.3.4. The effect of DVB on phosphoric acid loss

Samples of 0% and 1% DVB were selected and washed with water in order to determine the

effect of DVB on the internal film structure. The SEM images of membranes demonstrate the

fact that in case of 0% DVB the grafted polymer is not interconnected, and although is it able to

take more acid compared to the cross-linked membranes, washing will lead to the formation of

voids inside the membrane (Figure 3.5). In contrast, the graft structure of 1%DVB membranes

forms interconnected polymer networks which resist against acid loss due to washing.

Figure 3. 5. Scanning electron microscope image of grafted ETFE films: left) 0%DVB grafted film right) 1%DVB grafted film.

The further investigation of these phenomena continued by selecting two membranes of 0% and

1% DVB, but with the same graft level. This time the effect of acid up-take was investigated by

measuring the acid amount with respect to the absolute amount of grafted polymer (Figure 3.6).

Additionally, two membranes of 0% and 1% DVB which had the same amount of doped acid

were selected. The effect of washing for the membranes with the same graft level showed that

the membrane with 1% DVB can take almost 50% less acid compared to the non-cross-linked

membrane. However, after washing the membranes with the same acid amount, it was observed

that the membrane with 0% DVB lost more than 50% of its acid, while the 1% DVB membrane

only lost less than 20% of its acid.

46

Acid up-take of membranes with the same graft level

Acid loss of membranes with the same intial acid level

0 1

0

100

200

300

0

100

200

300

Ph

osph

oric a

cid

lo

ss (

%)

Ph

osph

oric a

cid

up

-ta

ke (

%)

DVB content (%)

Figure 3. 6. The effect of DVB on acid up-take and acid loss of grafted membranes with the same initial conditions.

The contact angle of acid doped membranes showed the underlying mechanism of resistance to

acid loss in cross-linked samples (Figure 3.7). The membrane with 0% DVB almost did not show

any hydrophobicity towards water. Surprisingly, the 1% DVB membrane showed a contact angle

more than 60°. This behavior can be explained through the lotusutos effect due to the formation

of nan-structured cross-linked acid pores in the membrane [176].

Figure 3. 7. The effect of cross-linking on contact angle of acid doped membranes: left) 0% DVB mebrane, right) 1% DVB

membrane.

47

3.3.5. The effect of DVB on mechanical properties of membranes

The tensile test was carried out for the acid doped synthesized membranes with 1:10 aspect ratio

at 100 mm/min elongation rate. The introduction of DVB up to 0.5% did not show a significant

contribution to the mechanical properties of membranes. At 1% DVB concentration the

membrane could show the highest elongation among the other samples because of the formation

of an interconnected network of grafted polymers. Although further increasing DVB

concentration led to increasing the strength of membranes, this increase also caused the decrease

of the elongation (Figure 3.8). Moreover, the physical appearance of membranes also depends on

crosslinking in such a way that after acid doping the 0% DVB membranes show a white

appearance due the interference of membrane voids with visible light, and consequently

scattering the light. However, the cross-linked membranes could maintain their transparency

after acid doping.

0 50 100 150 200 250

0

5

10

15

20

25

Str

ess (

Mpa

)

Strain (%)

0% DVB

1% DVB

2% DVB

Figure 3. 8. The tensile test results for acid doped membranes with 0%, 1% and 2% DVB content.

3.3.6. Ionic conductivity of grafted membranes

The ionic conductivity of grafted membranes was measured by means of a 4-probe conductivity

cell placed in a special apparatus for controlling temperature and relative humidity of membranes

during the measurements. Three membranes were studied, 0% DVB content with 30% graft

level, 1% DVB with same graft level as 0% DVB membrane, and 1% DVB with 50% graft level.

48

These membranes were tested at 80°C, 100°C and 120°C and varying relative humidity ranging

from 10% to 50% (Figure 3.9). Although the membranes were not washed, the ionic conductivity

of all membranes changed drastically by changing the RH. Additionally, all of the conductivity

values increased as the operating temperature rose. The maximum ionic conductivity of 75

mS/cm2 belonged to 1% DVB membrane with 50% graft level. Interestingly the 0% DVB could

also reach a 60 mS/cm2 conductivity, which is due to the high phosphoric acid up-take.

10 20 30 40 50

10

20

30

40

50

60

70

80

Ionic

co

nd

uctivity a

t 12

0°C

(m

S/c

m^2

)

Relative humidity (%)

0% DVB 30% GL

1% DVB 30% GL

1% DVB 50% GL

10 20 30 40 50

10

20

30

40

50

60

70

80

Ionic

co

nd

uctivity a

t 10

0°C

(m

S/c

m^2

)

Relative humidity (%)

0% DVB 30% GL

1% DVB 30% GL

1% DVB 50% GL

10 20 30 40 50

10

20

30

40

50

60

70

80

Ionic

co

nd

uctivity a

t 80

°C (

mS

/cm

^2)

Relative humidity (%)

0% DVB 30% GL

1% DVB 30% GL

1% DVB 50% GL

Figure 3. 9. The proton conductivity of ETFE-g-PVP membranes at different relative humidity and temperature.

3.3.7. Fuel cell performance of ETFE-g-PVP membranes

The fuel cell performance of membranes with 0% DVB and 30% graft level, and 1% DVB and

50% graft level was measured at 1 atmosphere gauge pressure and 50%RH at different

temperatures (Figure 3.10). Surprisingly, the fuel cell performance of cross-linked membrane

under all conditions was much better than the non-cross-linked membrane despite their similar

49

proton conductivities. This is because of the fact that the phosphoric acid inside the membrane,

due to interaction of phosphoric acid with gas diffusion layer, interferes with the three phase

boundary mechanism of electrodes [177]. Therefore, it can be speculated that the reason for the

improvement of cell performance is the capability of cross-linked membrane which withholds

phosphoric acid inside the membrane.

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

Fuel cell performance at 120°C

0% DVB voltage

1% DVB voltage

0% DVB power

1% DVB power

Cell current (mA/cm^2)

Ce

ll vo

ltag

e (

V)

0

30

60

90

120

Po

we

r (m

W/c

m^2

)

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

Fuel cell performance at 100°C

0% DVB voltage

1% DVB voltage

0% DVB power

1% DVB power

Cell current (mA/cm^2)

Ce

ll vo

ltag

e (

V)

0

30

60

90

120

Po

we

r (m

W/c

m^2

)

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

Fuel cell performance at 80°C

0% DVB voltage

1% DVB voltage

0% DVB power

1% DVB power

Cell current (mA/cm^2)

Ce

ll vo

ltag

e (

V)

0

30

60

90

120

Po

we

r (m

W/c

m^2

)

Figure 3. 10. Fuel cell performance of ETFE-g-PVP at 50%RH, 1atm and different temperatures.

3.4. Conclusion

Cross-linked ETFE based membranes were synthesized via radiation induced grafting of 4VP

with varying DVB concentration successfully, and were further modified by means of

subsequent doping with phosphoric acid. The obtained membranes were characterized for graft

level, acid up-take, mechanical properties and fuel cell performance. It was observed that the

introduction of 1% DVB to the structure of grafted polymer optimizes ETFE-g-PVP membranes

50

in terms of mechanical properties, proton conductivity, and power output. It is speculated that

modifying the nano-structure of cross-linked membrane is the reason for its hydrophobicity

which eventually led to a more efficient formation of the three-phase boundary between

membrane, electrode, and reactant gases.

51

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